The present invention is concerned with improvements to continuously variable transmissions which are able to provide infinite speed reduction (“geared neutral”).
FIG. 1 represents, purely by way of example and in highly schematic form, one such transmission. Box 10 represents a variator—that is, a device having a rotary variator input 6, a rotary variator output 8, and a mechanism for transferring drive between the two at a speed ratio (the “variator ratio”) which can be continuously (steplessly) varied over a finite range. Variators of numerous different types are known in the art. The variator may for example be a toroidal race rolling traction device as described in Torotrak (Development) Ltd's published UK application GB2423122 and its PCT counterpart WO2006084905. Box 12 represents an epicyclic (planetary) gear. The general construction of such gearing is well known and will not be described herein beyond noting that it comprises, in conventional manner, a central sun gear engaging with planet gears which are mounted upon a carrier and engage in their turn with an outer annular gear. The carrier is coupled through gearing R1 to a rotary input 14 of the transmission. The sun gear is coupled through the variator 10 and gearing R2 to the same transmission input 14. The annular gear is coupled via gearing R3 to a rotary output 16 of the transmission.
The transmission input 14 would typically be coupled to some source of rotary drive, such as the internal combustion engine of a motor vehicle. The transmission output 16 would be connected to some point of power usage, such as the driven wheels of the vehicle. The transmission serves to transfer drive between the two. The ratio of transmission output speed to transmission input speed is a function of the variator ratio, and is thus continuously variable.
Gearing in such a transmission is typically designed such that there is a certain variator ratio (the “geared neutral variator ratio”) at which the speeds of the carrier and sun gears cancel each other out, leaving the annular gear, and the transmission output 16, stationary, despite the fact that they remain mechanically coupled to the rotating transmission input 14. This is the infinite speed reduction referred to above, and transmissions having this facility are sometimes referred to as “infinitely variable transmissions”. Variator ratios to one side of the geared neutral variator ratio provide rotation of the transmission output 16 in one direction (e.g. forward drive for a motor vehicle). Variator ratios to the other side of geared neutral provide transmission output rotation in the opposite direction (reverse drive).
If one defines the speed ratio of the transmission to be the rotational speed of its output divided by that of its input, and takes rotation in one direction to be positive and rotation in the other direction to be negative, then speed ratio is positive when the transmission input rotate in the same direction, and negative when they rotate in opposite directions.
Other vehicle transmissions typically require a clutch or other means to mechanically de-couple the engine from the wheels when the vehicle is brought to rest, and to accommodate an initial mismatch of speeds between engine and transmission during vehicle launch. An infinitely variable transmission, however, makes it possible to halt the vehicle, and to move off from rest, without need of any such launch device, merely by appropriate adjustment of variator ratio.
It should be noted that the particular layout represented in FIG. 1 is presented merely in order to illustrate the general principles and is by no means the only arrangement suitable for implementing an infinitely variable transmission.
A problem can arise in relation to changes of engine speed. Suppose that the vehicle is stationary, the transmission is in geared neutral and the engine is idling. The driver then abruptly raises engine speed, in preparation for vehicle launch, but does not yet adjust the variator ratio. Since the ratio remains at its neutral setting, the driver's expectation is that no torque will yet be exerted on the vehicle wheels. However, inertias within the transmission must accelerate along with the engine. These inertias are mechanically coupled to the transmission output 16. The torque required to accelerate the inertias is reacted partly to the transmission output, and consequently a torque is briefly experienced at the vehicle wheels, potentially causing the vehicle to jerk forwards or backwards.
Another problem can arise if the transmission is not set precisely to geared neutral when the driver requires it, in which case unwanted creep of the vehicle may result.